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Coastal ProcessesSand Dunes Sand dunes are formed by wind blowing over the surface of the beach and drying out the sediments. As the sand dries the wind begins picking up sand grains and carrying them in-land, this is known as saltation. The wind picks up grains moves them forward and then due to gravity drops them. The dropping of the sand grains forces other sand grains to be picked up and carried further inland. It's like when a raindrop hits a puddle and forces droplets of water up into the air. The sand grains come from the beach and the origin of the sand grains is discussed in the beaches section. The sand dune system starts at the Strandline, which is found at the high water mark, this is where debris is deposited at high tides. The debris and vegetation traps wind blown sediment, as this sediment builds up an embryo dune is formed; this is the first dune type in the system. Embryo dunes are not stable and they can be washed away by high tides. As the embryo dune is colonised by salt tolerant plants it decreases the chance of it being washed away. Species, which can be found here, are sea rocket (Cakile maritime) and prickly saltwort; these are very special plants as they can cope with harsh conditions e.g. high salinity, low nutrients. This vegetation continues the formation of the embryo dune. As the vegetation grows, it traps more wind blown sand and grows taller in relation to the amount of sand, which is deposited. This stops the vegetation being buried. Embryo dunes can be 1-2 metres in height. These embryo dunes are less frequently covered by the tide and waves and subsequently more plant species are able to grow. As the dune height increases the vegetation changes and the embryo dune becomes a fore dune or yellow dune.
Yellow dunes are also known as mobile dunes this is because the morphology or shape changes. On the mobile dune the vegetation types change because the colonising vegetation on the embryo dunes find it hard to reach the water table as the height of the dune increases. Types of vegetation, which can be found on the mobile dune, are ragwort (Senecio jacobaea) and marram grass (Ammophila arenaria). These dunes are less salty than the embryo dunes as rain washes out some of the salt. The fore dunes can be up to 5 metres in height. The mobile dunes can decrease the amount of sand, which gets to the grey dunes further inland. This means that vegetation, which cannot survive burial, can begin to grow on these dunes. Sand continues to be blown from the beach into and over these dunes and the mobile dunes appear to move backwards.
Grey dunes or fixed dunes have a better water holding capacity and increased nutrient availability therefore more plant species can grow on them. Fixed dunes are completely covered by vegetation; marram grass is replaced by other grasses like red fescue (Festuca rubra). Fixed dunes vegetation can then change from grasses to woody plants like sea buckthorn (Hippophae rhamnoides).
Within dune systems are dune slacks these are areas, which have been eroded by the wind. Dune slacks are a lot lower than dunes and may have been eroded as low as the water table level. Due to water being available dune slacks have very different vegetation to the actual dunes. The types of plant species, which can be found, are sea club-rush (Bolboschoenus) or reed canary grass (Phalaris arundinacea), up to 78 different plant species have been found in dune slacks.
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